Research On Voltage Control Strategy Of DC Distribution Network

With the development of society,the problem of small transmission capacity and poor power quality of traditional AC power distribution method is becoming increasingly noticeable.The DC power distribution method promotes its own rapid development because of its suitability of connection between distributed generation and grid,high power quality,large power supply capacity,low line cost and high reliability.Voltage stability is an important prerequisite for the safe and stable operation of DC distribution systems.A good voltage control strategy is urgently needed to deal with the voltage instability caused by the power fluctuations of the system.The voltage control in the DC distribution network is mainly accomplished by the converter station.The voltage control system based on the converter station can be divided into three levels: system level control,converter level control,and converter valve level control.Firstly,the functions and relationships of all control levels are described,and the foundation for the following research on voltage control strategies is established.In order to improve the transient performance when the system power fluctuates,a control method that can provide virtual inertia to the system is designed in the process of calculating the power command value of each converter station at the system level control.Simulation results show that the designed control method can effectively improve the inertial characteristics and transient response of the system.In order to solve the problems,such as insufficient capacity of single converter station and constraint between voltage control and power distribution,when the traditional droop control is adopted to calculate the power command value of each converter station and accomplish voltage coordination control between stations as the system level control.An improved droop control strategy,considering the power limit and the voltage regulation range of the converter station,is designed.Simulations show that the proposed control strategy can effectively improve the power distribution performance between converter stations and relieve the problems of constraint between voltage control and power distribution.Aiming to solve the problem of voltage regulation deviation of the droop control,a method to eliminate the voltage regulation deviation is proposed,which is combined with the improved droop control strategy as the Voltage coordination control between stations in this paper,The simulation results show that when the voltage control strategy is used to calculate the transmission power command value of the converter station to perform coordinated control of the voltage between the converter stations,not only the power distribution performance between the converter stations can be improved,but also voltage regulation deviation can be eliminated.Aiming at the shortcomings of slow response speed and poor stability when the traditional dual closed-loop PI controller is used as converter level controller,a sliding mode controller is designed,because of its characteristics of fast response and strong robustness.Simulation results show that the designed sliding mode controller can speed up the converter station to track the power command value,improve transient characteristics and anti-jamming capability.Combining the above-mentioned virtual inertial control,the voltage coordination control between the converter stations,and the sliding mode controller,the voltage control strategy of the DC distribution network based on the converter station is formed.In view of the problem that the system voltage stability is still not controlled when the transmission power of the converter station reaches the limit when the system power fluctuation is large,a voltage control strategy based on distributed generation and secondary load switching is designed.The simulation verifies the effectiveness of the designed control strategy.